1. Field of the Invention
The present invention relates to case of a battery, and more particularly, to a case of a battery which hermetically seals and surrounds a battery body that generates current.
2. Description of the Related Art
According to the kind of electrolyte used, a lithium secondary battery can be classified as a lithium ion battery which uses a liquid electrolyte or a lithium ion polymer battery which uses a solid electrolyte.
The lithium ion polymer battery includes a battery body for generating current, which is made by stacking a cathode, an anode and a separator. An electrode terminal serving as an electrical path to the outside is connected to the battery body. Parts of the battery body and the electrode terminal are hermetically sealed in an insulating case. The case of the lithium ion polymer battery hermetically seals the battery body such that thermal adhesive material layers in the innermost layer of the battery body are adhered to each other.
FIG. 1 is an exploded perspective view showing an example of a conventional lithium ion polymer battery. Referring to FIG. 1, the lithium ion polymer battery includes an electrode assembly 11 having a cathode, an anode and a separator, and a case 12 surrounding the electrode assembly 11 to hermetically seal the same. Electrode terminals (or leads) 14 and 14xe2x80x2 serving as an electrical path for conducting the current generated in the electrode assembly 11 to the outside are installed such that a predetermined length of the same is exposed outside the case 12.
The case 12 is an insulating film having a laminated structure shown in FIG. 2. Referring to FIG. 2, the case 12 is constructed such that a poly(ethylene-co-acrylic acid)(EAA) layer 21 which is a thermal adhesive material layer is formed as the innermost layer with respect a to battery body 20, a polyethylene (PE) layer 22, a nylon layer 23, an EAA layer 21xe2x80x2 and an aluminum layer 24 are sequentially laminated thereon, and then a nylon layer 23xe2x80x2 is laminated as the outermost layer. Here, the PE layer 22 increases the insulation and adhesion effects. The EAA layer 21xe2x80x2 prevents the case 12 from swelling due to an electrolytic solution. The aluminum layer 24 prevents permeation of moisture and loss of an electrolyte solution. The nylon layer 23 prevents cracks from being generated at the outer layers and facilitates the formability of the inner layers.
The above-described case 12 hermetically seals the battery such that the electrode assembly 11 is inserted into the case 12, with only parts of the electrode terminals 14 and 14xe2x80x2 being exposed. Then, heat and pressure are applied to the case 12 to hermetically seal the battery such that thermally adhesive materials of the edges in the upper and lower case bodies are adhered to each other.
However, during heat-fusion of the edges of the upper and lower case bodies, if the temperature and pressure are so high that too much time is required in the heat-fusion, an EAA layer, which is a polymer layer on the case surface is melted, so that anodic and cathodic leads of the battery are brought into contact with aluminum, which is a case material, to cause a short circuit, resulting in degradation in assembly efficiency. Also, in the case where a micro short circuit occurs, the performance of the battery may be degraded.
To solve the above problems, it is an object of the present invention to provide a battery case capable of preventing a short circuit of the battery during heat-fusion.
To accomplish the above objective, the present invention provides a case of a battery for enclosing and hermetically sealing a battery body which generates current, the case including a first polyethylene terephthalate layer which is an innermost layer in view of a battery body and which is adhered to other part thereof when sealing the battery body, a polymer layer formed on the first polyethylene terephthalate layer, a second polyethylene terephthalate layer formed on the polymer layer, and a polypropylene layer which is an outermost layer in view of the battery body and formed on the second polyethylene terephthalate layer.
The polymer layer is preferably at least one selected from the group consisting of polytetrafluoroethylene (PTFE) resin, polyimide resin and silicon resin. This is because these materials have excellent heat resistance, oxidation resistance and chemical resistance and have low air permeability, thereby suppressing permeation of moisture and loss of an electrolytic solution. Also, the thickness of the polymer layer is preferably in the range of 10 to 50 xcexcm.